Electroplating of zinc and electrolyte therefor



Patented Oct. 21, 195 2 ELECTROPLATING OF ZINC AND ELEC- TROLYTETHEREFOR Allan E. Chester, Highland Park, 111., assignor to Poor &Company, Chicago, 111., a corporation of Delaware No Drawing.

Application January 26, 1951,

' Serial No. 208,079

9 Claims. 1

This invention relates to a new and improved method of electroplatingzinc from an acid zinc lioath and to a new and improved electrolytethere- One of the objects of the invention is to provide a new andimproved type of electrolyte for electroplating zinc from acid zincplating baths.

Another object of the invention is to provide anew and improved methodof electroplating zinc wherein electroplated articles having a brightinto the electrolytes certain chemical compositions or derivativesthereof which greatly assist in the deposition of bright zinc plates.

These and other objects and features of the invention are accomplishedby incorporating into acid zinc sulfate electrolyte baths the product ofthe reaction of thiourea with zinc oxide and an aldonic acid, preferablygluconic acid. When zinc is electroplated from such baths onto thecustomary base materials. the resultant products have a greatly improvedbrightness of finish.

The invention will be illustrated by the following examples showing amethod for preparing the addition agent and the preparation of theplating bath according to the invention. It will be understood that theinvention is not limited to the disclosure in the examples.

Emample I Thiourea, an aqueous solution of 50% gluconic acid and zincoxide are mixed together in the following amounts:

Pounds Thiourea 9.16 Gluconic acid (50% concentration) 575.00 Zinc oxide(less than 0. lead) 63.845

In mixing the foregoing ingredients, the gluconic acid was first addedto a stainless steel tank and mixed with the thiourea with agitation.The zinc oxide was then added slowly at the rate of 50 pounds every 20minutes. No heat is required since the reaction takes placeexothermically. When the addition is complete and foaming subsides thebatch is brought to a temperature of 200 F. and maintained at thistemperature for 20 minutes. While regulating the temperature between 185F. and 195 F. the batch is cooked approximately 2 hours or until theviscosity of 96 ml. (milliliters) of the product is 75-77 seconds. Theviscosity determination is made with a 100 2 ml. pipette No. 715calibrated to deliver 100 ml. of water in 45 seconds at 20 C.

The reaction mixture is tested at intervals. When the viscosity isseconds or less, 1 hour is allowed between tests. When the viscosity is'70 seconds tests are made every half hour. When the viscosity is 72seconds tests are made every 20- minutes. At 65 seconds the pH of a 10%aqueous solution of the reaction mixture should be 6.0 to 6.5. If the pHis below 6.0 more zinc oxide is added to bring it up to the requiredrange. 'If the pH is above 6.5 more gluconic acid is added. At 75-77seconds viscosity the reaction mixture is poured into trays in layers ofabout A; to inch thickness. After 24 hours the product is placed on agalvanized screen rack to air dry. When'it is dry it is milled to breakit up into solid particles of the desired size. The resultant product isreadily soluble in acidic aqueous solutions having a pH within the rangeof 1.5 to 6.0.

Example II This example illustrates the preparation of acid zinc platingbaths according to the invention. The composition prepared as in ExampleI when added in proportions of 1 to 3 ounces per gallon to an acid zincsulfate plating bath containing to 450 grams per liter of zinc sulfate(ZHSO4.'7H2O) produces a brightening eifect at pI-Is within the range of1.5 to 6.0. This plating bath will operate near a pH of 6.0 without theformation of Zinc or iron precipitates and produces high specularreflectivity of the plated metal going up to mirror brightness,depending upon the quantity of the zinc I gluconatethiourea compositionemployed. Excellent deposits are obtained over a wide range of currentdensities from 5 amperes per square foot to 5000 amperes per squarefoot, the current density employed being dependent upon the agitationavailable. 1

The compositions are especially effective in the electrodeposition ofzinc from acidic baths in which the zinc is principally present as zincsulfate. The zinc sulfate baths can also contain additions of zincchloride, preferably not more than 20 to 100 grams per liter.

In plating from acid zinc baths according to the invention it issometimes desirable to employ a small amount of aluminum sulfate in thebath, for example, to 2 ounces per gallon. The aluminum sulfate forms afilm of colloidal alumina in the vicinity of the anode and preventsexcessive acid attack on the zinc anode. Another optional ingredientwhich may be added to the bath is sugars, such as dextrose, whichapparently produce a refinement in the crystalline structure of the zincand have some whitening effect. Still another optional ingredient issodium fluoride which acts as a whitening and grain refining agent atthe cathode and apparently causes more uniform solution of zinc at theanode while tending to prevent sludge formation. Other ingredients suchas sodium acetate may also be employed but are not required.

Electroplating baths of the type described herein may be used as bathsfor plating piecework, that is, for batch operations, or may be employedin continuous plating operations.

If the electroplating baths are used in continuous plating operationsthe makeup bath may be prepared as described in Example II andthereafter the gluconate which is lost due to drag-out is replenished byadding to grams per liter of zinc gluconate at intervals aspredetermined by tests of the bath. The zinc gluconate may be preparedas described in Example I by omitting the thiourea. It can also beprepared as described in my copending application Serial No. 180,500,filed August 19, 1950.

The plating baths may be operated at conventional temperatures withinthe range of 70 F. to 160 F., preferably not higher than 140 F.

'In the preparation of the compositions employed in the presentelectrolyte other aldonic acids may be used instead of gluconic acid, asfor example, mannonic, arabonic, galactonic and xylonic. The inventioncontemplates the use of one or more of these acids or mixtures thereof,including mixtures of the lactone forms of these acids. Commercialgluconic acid is available as a 50% aqueous solution of approximately99% gluconic acid and 1% glucose. The delta gamma lacton form ofgluconic acid is particularly effective in producing compositions whichact as brighteners for acid zinc plating baths. It is significant thatthe same brightening effects cannot be obtained by adding zinc gluconateand thiourea separately to baths of the type described.

The relative proportions of the reactants employed is subject tovariation. For the purpose of the invention it is preferable todetermine the proportions by the pH test given in Example I. Thereaction between zinc oxide and gluconic acid to form zinc gluconaterequires approximately 1 mole of zinc oxide to 2 moles of gluconic acidbut an excess of either reactant may be present in the final product.The manner in which the thiourea reacts is not clearly understood andthe quantities employed may be varied but preferably correspond to 0.025to 1.0 mole of thiourea per mole of gluconic acid. The preferred molarratio of thiourea to zinc oxide to aldonic acid is approximately 1:6:12.The molar ratio of thiourea to aldonic acid should be at least 1:24 andpreferably not more than 1:6. The molar ratio of zinc oxide to aldonicacid should be at least 1:4 and prefer.- ably about 1:2.

In addition to the brightening effect of the compositions of theinvention in acid zinc plating baths, these compositions have apronounced anti-burn effect. Hence, they make it possible to operate thebaths at very high current densities with less burning of the platedarticle.

The term acid zinc sulfate bath is used herein to describe acidicplating baths in which the zinc is present principally as zinc sulfatebut can also be present in minor amounts in one or more other forms.

The compositions disclosed herein are further disclosed in my copendingapplication Serial No. 199,582 filed December 6, 1950.

The invention is hereby claimed as follows:

1. An aqueous acid zinc sulfate plating bath comprising an acid zincsulfate electrolyte and a reaction product of thiourea, zinc oxide andan aldonic acid.

2. An aqueous acid zinc sulfate plating bath comprising an acid zincsulfate electrolyte and a reaction product of thiourea, zinc oxide andgluconic acid.

3. An aqueous acid zinc sulfate plating bath comprising an acid zincsulfate electrolyte and a reaction product of thiourea, zinc oxide andan aldonic acid in proportions of about 1 to 3 ounces per gallon ofplating bath.

1. An aqueous acid Zinc sulfate plating bath comprising an acid zincsulfate electrolyte and a bath soluble reaction product of thiourea,zinc oxide and gluconic acid in proportions of about 1 to 3 ounces pergallon of plating bath.

5. In a process for the electrodeposition of zinc the step whichcomprises electrodepositing zinc from an acid zinc sulfate plating bathin the presence of a product of the reaction of thiourea, zinc oxide andan aldonic acid.

6. In a process for the electrodeposition of zinc the step whichcomprises electrodepositing zinc from an acid zinc sulfate plating bathin the presence of a product of the reaction of thiourea, zinc oxide andgluconic acid.

7.1m a process for the electrodeposition of zinc the step whichcomprises electrodepositing Zinc from an acid zinc sulfate plating bathin the presence of a product of the reaction of thiourea, zinc oxide andan aldonic acid, said product being present in proportions of about 1 to3 ounces per gallon of plating bath.

8. The process for continuously electrodepositing zinc which comprisesdepositing zinc from an acid zinc sulfate plating bath in the presenceof about 1 to 3 ounces per gallon of plating bath of the product of thereaction of thiourea, zinc oxide and gluconic acid and replenishing theconsumed gluconate by continuously adding zinc gluconate to the bath.

9. The process for electrodepositing zinc which comprises depositingzinc from an acid zinc sulfate plating bath containing about 1 to 3ounces per gallon of the plating bath of the bath soluble product of thereaction of about 9 parts by weight of thiourea, about 575 parts byweight of gluconic acid (50% concentration) and about 64 parts by weightof zinc oxide, said product having a viscosity of about to 77 seconds asdetermined on a ml. pipette calibrated to pass 100 ml. of Water in 45seconds at 20 C.

ALLAN E. CHESTER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

1. AN AQUEOUS ACID ZINC SULFATE PLATING BATH COMPRISING AN ACID ZINCSULFATE ELECTROLYTE AND A REACTION PRODUCT OF THIOUREA, ZINC OXIDE ANDAN ALDONIC ACID.